Japanese Patent Application Publication No. 2001-254624 (Literature 1) discloses an air-head stratified scavenging two-stroke engine. This two-stroke engine has a piston, a cylinder in which the piston is housed so as to be able to reciprocate, a crankshaft connected to the piston via a con rod, and a crankcase in which the crankshaft is housed so as to be rotatable. This two-stroke engine also has formed therein an air-fuel mixture passage for introducing an air-fuel mixture (a mixture of fuel and air) into the crankcase, a scavenging passage that extends between a scavenging inflow port opened into the crankcase and a scavenging port opened into the cylinder, and an air passage connected to an intermediate position of the scavenging passage.
In this two-stroke engine, negative pressure that is generated in the crankcase acts on the scavenging passage through the scavenging inflow port at the time of an up stroke of the piston, whereby the air is introduced from the air passage to the scavenging passage. The air introduced to the scavenging passage is introduced into the cylinder before the air-fuel mixture at the time of a down stroke of the piston. An air layer is formed between combustion gas and the air-fuel mixture when the combustion gas is scavenged from within the cylinder. The formation of this air layer prevents blow-by of the air-fuel mixture, reducing the emission of unburned gas.
Another air-head stratified scavenging two-stroke engine is disclosed in WO98/57053 (Literature 2). In this two-stroke engine, an air passage is connected to a scavenging port by a piston at the time of the up stroke of the piston. Consequently, air fills up a scavenging passage from the scavenging port. This type of configuration can prevent an air-fuel mixture from remaining in the vicinity of the scavenging port when the air fills up the scavenging passage.